For conventional low level exhaust gas recirculation (EGR), exhaust gas expelled from the cylinders of an internal combustion engine may be collected in a collector of an exhaust manifold. A fraction of the collected exhaust gas (e.g. 5% to 10%) may then be routed from the exhaust manifold through a control valve back to an intake manifold of the engine, where it may be introduced to a stream of ambient air/fuel (A/F) mixture. The remaining fraction of exhaust gas in the exhaust manifold, rather than being recirculated and recycled, generally flows to a catalytic converter of the exhaust system and, after treatment therein, may be expelled to the atmosphere.
EGR has a history of use in both diesel and spark-ignition engines, and affects combustion in several ways. The combustion may be cooled by the presence of exhaust gas, that is, the recirculated exhaust gas may absorb heat. The dilution of the oxygen present in the combustion chamber with the exhaust gas, in combination with the cooler combustion, may reduce the production of mono-nitrogen oxides (NOx), such as nitric oxide (NO) and nitrogen dioxide (NO2). Also, when exhaust gas is recirculated, less air may be breathed by the engine, which may reduce the amount of exhaust gas produced. Additionally, EGR may reduce the need for fuel enrichment at high loads in turbocharged engines and thereby improve fuel economy.
EGR which uses higher levels of exhaust gas may further increase fuel efficiency and reduce emissions of spark-ignition engines. However, with higher levels of exhaust gas, engines may face challenges related to EGR control and tolerance, which may reduce the expected fuel efficiency improvement. Challenges related to EGR control may be understood to include reducing a variability of the exhaust gas, particularly composition and distribution. If a variation in the exhaust gas introduced to an engine is too random, fuel efficiency improvements may suffer. Challenges related to EGR tolerance may be understood to include increasing an engine's ability to process higher levels of exhaust gas without adversely affecting performance, particularly fuel economy. Thus, even if EGR control and tolerance may be satisfactory for engine operation at low levels of EGR, an engine may need additional modifications in structure and operational conditions to accommodate higher levels of EGR without adversely affecting engine performance.
More recently, an engine configuration has been proposed with one or more cylinders of an engine dedicated to expelling exhaust gas for EGR. Such cylinders may be referred to as dedicated EGR, or D-EGR, cylinders. Dedicated EGR cylinder(s) may operate at a broad range of equivalence ratios since their exhaust gas is generally not configured to exit the engine before passing through a cylinder operating at, for example, a stoichiometric or near stoichiometric air/fuel ratio. This may allow the dedicated EGR cylinder to be run rich to produce higher levels of hydrogen (H2) and carbon monoxide (CO) which, may in turn, enhance flame speeds, combustion, and knock tolerance of all the cylinders.